1. Filed of the Invention
The present invention relates to a resistor circuit in which a resistor has a reduced TCR (Temperature Coefficient of Resistance).
2. Description of the Related Art
FIG. 6 shows a conventional constant-current circuit. A resistor 5 is connected to an emitter terminal of a transistor 3 for detecting a current which is fed back to an operational amplifier 4. The operational amplifier 4 controls the transistor 3 so that the voltage of a connecting point between the emitter terminal and the resistor 5 corresponds to a constant-voltage Vc. Thus, the circuit keeps a current which flows into a load 6 constant.
When such a circuit is constructed by a so-called hybrid IC (Integrated Circuit), a thick-film resistor is generally used as the resistor 5. However, when sheet-resistivity of the thick-film resistor is approximately less than 1.OMEGA./.quadrature., the thick-film resistor tends to behave metallically. More specifically, the TCR of the thick-film resistor becomes more than +500 ppm/.degree.C. In this case, the resistance of the resistor 5 changes in accordance with variations in ambient temperature. Therefore, the voltage which is fed back to the operational amplifier 4 is changed because of the resistance variation, and this voltage change will vary the current. Therefore, the circuit can not keep the current constant.
A conventional electrode structure for the resistor 5 is shown in FIG. 7. The TCR of a resistive film 2 is comparatively low (approximately +150 ppm/.degree.C.), and its resistance is high. The resistive film 2 is formed on a wide area between linear conductive films 1A and 1B to make resistance between the conductive films 1A and 1B. A terminal 20 shown in FIG. 7 is connected to the emitter terminal shown in FIG. 6, and a terminal 21 is connected to the operational amplifier 4. However, even such an electrode structure has not been able to sufficiently lower the TCR of the resistor 5 to enable constant current in changing ambient temperatures.